In the field of frequency-to-digital converters, it is a practice to count, over a predetermined sample time T.sub.S, the number of cycles N.sub.I of the input signal of unknown frequency F.sub.I. This input signal count is then divided by the time T.sub.I of the number of cycles N.sub.I to provide the determined frequency F.sub.I = N.sub.I .div. T.sub.I that is then provided in digital form in a binary code.
To provide an accurate realization of the frequency F.sub.I, a relatively high frequency asynchronous clock signal of a predetermined frequency F.sub.C and of a predetermined number of counts N.sub.C of clock signal pulses is used to establish the time T.sub.C, i.e., T.sub.C = N.sub.C F.sub.C. However, because the clock signal is asynchronous with respect to the input signal, the time differences between the integral number of input signal counts N.sub.I and the integral number of clock signal counts N.sub.C over the sample time T.sub.S during which the input signal is to be sampled must be separately determined and then added to and/or subtracted from the time T.sub.C to provide an accurate determination of the time separation between the first and the last of the counted input signal pulses N.sub.I, i.e. the lead, i.e., .DELTA.T.sub.L, of the leading first input signal pulse re the corresponding clock signal pulse is added to the time T.sub.C while the lag, i.e., .DELTA.T.sub.T, of the lagging corresponding clock signal pulse re the last input signal pulse is subtracted from the time T.sub.C, i.e., EQU F.sub.I = N.sub.I .div. [T.sub.C + (.DELTA.T.sub.L - .DELTA.T.sub.T)]
where the time EQU T.sub.I = T.sub.C + (.DELTA.T.sub.L - .DELTA.T.sub.T)
and the sample time EQU T.sub.S = T.sub.C + .DELTA.T.sub.L.
the determination of these correcting time durations, .DELTA.T.sub.L and .DELTA.T.sub.T, if of extremely short durations such as when the frequencies F.sub.I and F.sub.C concerned are in the megacycle (MHz) range, are extremely difficult to accurately determine. Such determinations require extremely accurate measuring techniques to provide the required accuracy in the determination of the frequency F.sub.I.